This invention pertains to the field of elevators, and in particular, to determining the absolute position and velocity of a moving elevator car.
To stop an elevator smoothly and level with a landing, the system must know when to initiate the stop, when to go into a leveling mode, and when to begin opening the elevator car doors. To perform these functions, it is necessary to know the exact position of the car at all times. Installations, especially high-rise installations, typically use a digital encoder known as a primary position transducer (PPT) to monitor the travel of the car in the elevator hoistway. The PPT is mounted in the machine room in a position that allows a steel toothed tape (xe2x80x9cselector tapexe2x80x9d) to be directly hitched to the frame of the car. As the car moves up and down the hoistway, the selector tape drives a sprocket or tape sheave which in turn drives the rotor of the PPT to provide a constant digital readout of the car position to within {fraction (1/64)}th of an inch.
Mounting a selector tape requires a sheave in the machine room and an idler sheave in the elevator pit to keep the tape from fluttering. The tape is alongside the elevator car within the hoistway and requires additional space for installation. Today, the design emphasis is on making the xe2x80x9cfootprintxe2x80x9d of the entire elevator system as small as possible, so as to maximize usable space in the building for whatever purpose is intended for the building itself. To this end, an alternate way of determining the car position is desired.
Briefly stated, a position reference system for an elevator car includes a laser that emits a beam that is reflected from a mirror. Either the laser or the mirror is in a non-moving position, while the other is fixed to the elevator car and moves with it. The laser beam is modulated at two frequencies, one of which provides a coarse position of the elevator car while the other provides a fine position of the elevator car. Position calibration occurs when the elevator car is stationary. When the elevator car begins to move, the coarse position is tracked while the fine position is determined from the higher of the two modulation frequencies. The absolute position of the moving elevator car is thus always known to a degree of accuracy depending on the higher modulation frequency.
According to an embodiment of the invention, a position reference system for an elevator car includes emission and response means for emitting electromagnetic radiation from a source, and causing a response from the response means when the electromagnetic radiation strikes the response means; modulation means for modulating the electromagnetic radiation at two different frequencies; means for determining if the elevator car is stationary; calibration means, when the elevator car is stationary and responsive to the modulation means, for calibrating the system when the elevator car is stationary to determine an initial position of the elevator car; coarse position means, responsive to the initial position of the elevator car and the modulation means, for determining a coarse position of the elevator car when the elevator car is moving; fine position means, responsive to the modulation means, for determining a fine position of the elevator car when the car is moving; and means, based upon the coarse position and the fine position, for determining an absolute position of the elevator car when the elevator car is moving.
According to an embodiment of the invention, a method for determining an absolute position of an elevator car includes the steps of emitting electromagnetic radiation from a source, and causing a response from a response device when the electromagnetic radiation strikes the response device; modulating the electromagnetic radiation at two different frequencies; determining if the elevator car is stationary; calibrating, responsive to the step of modulating and the step of determining, the system when the elevator car is stationary to determine an initial position of the elevator car; determining, responsive to the initial position of the elevator car and the step of modulating, a coarse position of the elevator car when the elevator car is moving; determining, responsive to the step of modulating, a fine position of the elevator car when the car is moving; and determining, based upon the coarse position and the fine position, an absolute position of the elevator car when the elevator car is moving.